1. Field of the Invention
The invention is related to systems and methods for estimating the response of rock formations in the Earth's subsurface to imparted electromagnetic fields in order to determine spatial distribution of electrical properties of the formations.
2. Description of the Related Art
Porous subsurface sedimentary rock formations are typically saturated with fluids as a result of having been deposited in a body of water during sedimentation. As a result, the fluids were initially entirely water. In some subsurface formations the water in the pore spaces has been displaced to some extent after sedimentation by hydrocarbons such as oil and gas. Thus, in some present day subsurface formations, the fluids in their pore spaces may be water, gas or oil, or mixtures of the foregoing.
Detection of formations having less than fully water-saturated pore space, that is, when oil or gas is present in the pore spaces, is of significant economic interest. Certain techniques for detection of such formations include determining existence of electrical resistivities in the subsurface that are anomalously high. The principle of such detection is based on the fact that the flow of electric current through a porous rock formation is related to the fractional volume of the pore spaces with respect to the total rock volume, the spatial configuration of the pore spaces and the electrical properties of the fluids filling the pore spaces. Brine-saturated porous rock formations, for example, are typically much less resistive than the same rock formations having hydrocarbons in some or all of the pore spaces, because brine is a relatively good electrical conductor while hydrocarbons are typically good electrical insulators.
Various techniques for measuring the electrical resistivity of subsurface rock formations are known in the art, for example, time domain electromagnetic survey techniques such as described in International Patent Application Publication No. WO 03/023452. Such techniques in general include imparting an electromagnetic field into the subsurface formations and measuring electric and/or magnetic fields induced in the subsurface formation in response to the imparted electromagnetic field. For such measurement techniques, the electromagnetic field may be imparted using an electric field transmitter, for example, by passing an electric current through a dipole electrode pair. Alternatively a magnetic field transmitter may be used, for example, passing an electric current through a wire loop or a plurality of such loops. The receivers used to detect the responsive electromagnetic fields may be dipole electrode pairs for measuring potential differences (electric field potential), or may be wire loops, pluralities of wire loops or magnetometers for measuring magnetic field amplitude and/or the time derivatives of magnetic field amplitude. The electric current used to impart the electromagnetic field may be controlled to provide a step change in the current. Step change in the transmitter current induces what are referred to as “transient” electromagnetic fields, and the responses measured by the receivers are related to transient response of the formations in the Earth's subsurface. Step change in the transmitter current may be obtained by switching the current on, switching the current off, reversing polarity, or combinations of the foregoing. A particularly advantageous form of transmitter current switching configuration used to impart the electromagnetic field is a so called “pseudo-random binary sequence” (PRBS) The following paper describes the use of PRBSs in electromagnetic exploration: Duncan, P. M., Hwang, A., Edwards, R. N., Bailey, R. C., and Garland, G. D., 1980, The development and application of a wide band electromagnetic sounding system using a pseudo-noise source, Geophysics, Vol. 45, pp 1276-1296.
In surveying an area of the subsurface using electromagnetic techniques, it is desirable to obtain signals corresponding to various distances (“offsets”) between the transmitter and receiver. In a typical survey implementation using PBRS transmitter current switching, a different bandwidth PRBS can be used for different ranges of offset. In one such example, for surveying formations below the bottom of a body of water, a receiver vessel may deploy a plurality of receivers in a selected pattern, such as a line array, on the water bottom. A separate transmitter vessel may deploy the transmitter on or at a nominal distance from the water bottom. The transmitter may be actuated and signals from the receivers recorded. Electromagnetic signals corresponding to various offsets may be obtained by moving the transmitter vessel, actuating the transmitter, and recording signals from the receivers, successively. The transmitter current is measured during actuation and the measurements thereof can then be transmitted to the receiver vessel for data quality control and processing.
To survey different areas of the subsurface below the water bottom, the receiver vessel may withdraw the receivers from the water bottom, move to a different location, and once again deploy the receivers on the water bottom in a different location. The above-described transmitter deployment, transmitter actuation and signal recording may then be repeated.
It is desirable to improve efficiency with which electromagnetic signals are obtained. One possibility is to actuate more than one transmitter at approximately the same time, and record the signals generated in response. Such technique could reduce the time taken to acquire a plurality of different offset electromagnetic signals. One technique for optimizing one or more signal acquisition parameters is described in International Patent Application Publication No. WO 2007/104949. Generally, the technique described in the foregoing publication includes selecting a transient switching frequency for the current applied to the transmitter that is inversely related to the square of the distance between the transmitter and the receivers (called “offset”). Such technique could be advantageously used with a technique that enables operating a plurality of electromagnetic transmitters essentially simultaneously.